Users of portable battery-powered communication devices are dependent of a fully functional device. More specifically, the users need to know exactly for how long their devices will remain functional, until the electric energy stored in the battery has been consumed and the battery has to be recharged. This is particularly true for users of mobile telephones. Hence, for the rest of this document a mobile telephone is used, in a non-limiting sense, for exemplifying the inventive portable communication device and method.
In order to determine a precise estimation of the remaining operational time of the mobile telephone, the user needs an accurate and well-functioning battery capacity indicator or “fuel gauge”. FIG. 1 illustrates a schematic mobile telephone 1 having a display 6, on which an icon 13 is presented as a battery capacity indicator. As shown in FIG. 1, the battery icon 13 indicates that approximately 25% remains of the initial battery charge. The mobile telephone 1 may have further display facilities for determining and indicating an estimated remaining time of operation, i.e. an estimation of the time left until the battery will have to be recharged.
Determining the remaining battery capacity basically includes two separate current measurements; one measurement for the current flowing into the battery (charging) and one measurement for the current consumed from the battery (discharging).
The charging current is often relatively easy to measure. A microprocessor (CPU) may read an A/D-converted signal, which is directly proportional to the current flowing through a small resistor. Since the microprocessor controls the charging process, it will also have access to all relevant data for calculating the total currents that has been supplied to the battery during a certain period of time.
Measuring the discharge current or current consumption, on the other hand, is much more difficult, particularly for advanced telephones with complex functionality and many operating modes. Traditionally, discharge current is measured by calculating the expected current consumption when the telephone is in different operating modes. Earlier mobile telephones basically had two operating modes only: talk mode and standby mode. For such mobile telephones, the current consumption in talk mode and standby mode, respectively, was measured once in a test laboratory environment and stored in memory in the telephone as a respective predetermined consumption value. In operation, the telephone would keep track of the time spent in talk mode and in standby mode, respectively, and subsequently calculate the total amount of current consumed from the battery by multiplying the respective operational times with the predetermined consumption values.
Such an approach is disclosed in U.S. Pat. No. 5,248,929, wherein a microprocessor in the mobile telephone regularly executes an interrupt-driven software routine (once every 100 ms), during which the momentary operational mode is determined. The predetermined consumption values are read from memory, and the resulting charge consumption value is added to an accumulated value, which in turn is used for determining remaining battery capacity and operational time in talk mode and standby mode.
Although providing an acceptable charge consumption estimation for a simplified scenario with only two operating modes, the approach described above has not proven applicable to more advanced telephones having a plurality of operational modes. For instance, the charge consumption of a contemporary TDMA (“Time Division Multiple Access”) telephone does not only depend on whether the telephone is in standby mode or talk mode; the charge consumption is affected by at least the following conditions in standby mode and talk mode, respectively:
Standby mode                Number of neighboring base stations        Paging frequency        Backlight        LCD icon mode on/off        Location update frequency        Top indicator        Accessory connected/disconnected        
Talk mode                Output power        Backlight        Band (900/1800/1900 MHz)        HR/FR/EFR (Half Rate/Full Rate/Enhanced Full—Rate), i.e. speech encoder mode        HF algorithm        DTX/no DTX (Discontinuous Transmission)        DRX/no DRX (Discontinuous Reception)        Accessory connected/disconnected        
Of the above, the output power in talk mode has a major influence on the battery charge consumption. For a telephone which operates in more than one frequency band, i.e. a multi-band telephone, the output power is different for different frequency bands. In view of these other parameters which will affect the charge consumption, a multi-band telephone will have a very large number of different operating modes. Previous approaches fail to provide accurate and still efficient charge consumption determination, and consequently there is an urgent need for an alternative way of determining battery charge consumption for a multi-band telephone.